In the known monitoring circuit according to U.S. Pat. No. 3,952,554 two yarn monitoring means are provided in each yarn feeding device. One of the two yarn monitoring means monitors the supplied yarn and is designated as the ingoing yarn monitoring means while the other of said two yarn monitoring means monitors the discharged yarn and is designated as the outgoing yarn monitoring means. The ingoing yarn monitoring means is operated when there is yarn breakage on the entry side or excessive tension of the ingoing yarn. The outgoing yarn monitoring means serves to detect yarn breakage on the discharge side, as well as a tension of the outgoing yarn which is too low, as occurs, for instance, when there is excessive yarn speed.
The two switches are in communication with a supply line via oppositely directed diodes of a first rectifier circuit and via an indicating lamp which is mounted on each yarn feeding device. The supply line is connected to an a.c. voltage source. When one of the two yarn monitoring means of a yarn feeding device is responsive, the corresponding switch is closed so that the rectifier diode of the first rectifier circuit which is associated with this switch is grounded via the switch. A current of half-wave shape respectively flows from the supply line via the indicating lamp and the rectifier diode toward ground during the half-wave of the a.c. voltage signal which is applied to the supply line for which the rectifier diode is conductive. Hence, when only one of the two yarn monitoring means is responsive, the indicating lamp is excited with current having a.c. voltage half-waves of only one polarity. However, when both yarn monitoring means are responsive at the same time, the indicating lamp is triggered with an a.c. voltage signal comprising both half-waves. The indicating lamp is dimensioned for this fault case in which it is triggered with the maximum current. This, however, has the result that when there is a usual fault case in which only one of the two yarn monitoring means is responsive, a relatively weak light signal is outputted by the indicating lamp. This makes it relatively difficult for the operator of the textile machine or knitting machine to locate that yarn feeding device in which the fault occurred, which in the final analysis led to the disconnection of the entire textile machine or knitting machine. It is difficult to detect the yarn supplying device which is failing, especially in textile machines or knitting machines of the type which is now in use and includes up to 144 yarn feeding devices. Moreover, the operator cannot judge by the light signal outputted by the indicating lamp whether a defect occurred in the respective yarn feeding device on the entry side, the discharge side or on the entry and discharge sides at the same time.
In view of this state of art, the present invention is based on the object of developing a monitoring circuit of the above-mentioned kind in such a way that the fault indication is improved by the indicating lamp of the yarn feeding device.